This invention relates to a method of producing a silicon nitride base sintered body which is high in mechanical strength at normal and high temperatures.
Silicon nitride base sintered bodies are chemically stable at both normal and high temperatures and, when properly sintered, possess high mechanical strength. Accordingly silicon nitride ceramics are suitable materials for bearings and the like and for engine parts represented by turbocharger rotors, and, furthermore, will have much applications in wide fields of industry including automobiles, machinery, chemical apparatus and aerospace devices.
Silicon nitride is rather inferior in sinterability so that it is difficult to accomplish good sintering of practically pure silicon nitride, Si.sub.3 N.sub.4, to obtain a sufficiently densified ceramic body. Therefore, it is usual to add a sintering aid such as MgO or a combination of Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3 to silicon nitride powder to be compacted and sintered. As is commonly accepted, sintering of the raw material containing such a sintering aid proceeds as a sort of liquid phase sintering through the medium of a liquid phase provided by the heated sintering aid. As a result, a glass phase which originates from the liquid phase will exist in the sintered ceramic body. The glass phase significantly and unfavorably affects important characteristics of the sintered body such as creep resistance, high temperature strength and oxidation resistance. Particularly when the glass phase in a silicon nitride base sintered body is relatively low in softening temperature, the sintered body is considerably degraded in its mechanical characteristics at high temperatures.
It is known to employ a hot press sintering method for accomplishment of good sintering of silicon nitride with reduced addition of sintering aid as the origin of an unfavorable glass phase. By this method it is possible to obtain sufficiently densified sintered bodies that exhibit good mechanical characteristics even at high temperatures. However, it is impracticable to produce sintered bodies of intricate shapes by this method. Also it is known to sinter silicon nitride with addition of aluminum oxide and aluminum nitride so as to produce a solid solution, viz. .beta.-sialon, to thereby incorporate the liquid phase originating from aluminum oxide into the silicon nitride particles under sintering. However, .beta.-sialon is very low in mechanical strength at normal temperature, though the strength does not greatly change by heating.